If you build a passive version of this project you want it to be air-tight so that dust won't escape. If you build an active system you need the unit to allow air to flow in so your vacuum can work. Lack of incoming air could damage the motor of your vacuum.
You can allow for air flow in one of two ways:
1) You can build a specialized opening that allows air to enter while the rest of the unit is air-tight. This would allow you to also use the system in a passive mode if your vacuum was unavailable.
2) You could allow spaces around the various penetrations into the system. This would allow air to flow in along multiple paths and requires no specialized openings. The downside being that it would be messy to use it in a passive mode as dust would be more likely to escape. I went with this method as I am a sloppy cutter.
A note: I'm entering this in the Craftsman contest so there will be blatant ass-kissing shots of the Craftsman tools I used for this project. Product placement will probably not sway the judges, but it can't hurt. :)